The subject matter disclosed herein relates generally to aircraft engines and, more specifically, to controlling a turbine clearance within an aircraft engine to facilitate more efficient operation of the aircraft engine during operations.
At least some known aircraft include an engine control system, sometimes referred to as a full authority digital engine control (FADEC). The FADEC is a system that includes a digital computer and its related accessories that control all aspects of aircraft engine performance. The FADEC receives multiple current input variables of the current flight condition including, for example, but not limited to, air density, throttle lever position, engine temperatures, engine pressures, and current values of other engine parameters. The inputs are received and analyzed many times per second. Engine operating parameters such as fuel flow, stator vane position, bleed valve position, and others are computed from this data and applied as appropriate to provide optimum engine efficiency for a given current flight condition.
The aircraft also typically include a flight control system, which may include a system typically referred to as a flight management system (FMS). The FMS is a specialized computer system that automates a wide variety of in-flight tasks, including the in-flight management of the flight plan. Using various sensors, such as, but not limited to, global positioning system (GPS), inertial navigation system (INS), and backed up by radio navigation to determine the aircraft's position, the FMS guides the aircraft along the flight plan. From the cockpit, the FMS is normally controlled through a Control Display Unit (CDU) which incorporates a small screen and keyboard or touch screen. The FMS transmits the flight plan for display on the EFIS, Navigation Display (ND) or Multifunction Display (MFD). The FADEC and FMS are separate system that in some cases may communicate current values of parameters.
Some known aircraft engines include a turbine including a hot section and a cold section. To improve fuel efficiency, thrust, and/or turbine life, at least some known engines attempt to control a distance or clearance between a tip of each turbine blade and a surrounding shroud to a minimum. However, a blade tip length, as measured from a rotor center, may increase in proportion to the square of an angular velocity of the rotor, and linearly with temperature. Both of such effects may be caused by increasing fuel flow during maneuvers such as climbs, certain acts in the descent/landing sequence, and/or evasive actions. Moreover, the blade tip length may increase more rapidly than the shroud expands during operation, especially during transient operations, such as those that require increased fuel flow. As such, during such operations, the blade tip may make contact with the shroud in a condition known as a rub.
At least some known aircraft engines use active clearance control to prevent rubs. Active clearance control, in at least some known embodiments, attempts to cause the shroud to expand linearly by bathing the shroud in hot air, based on similar physical properties that cause the blade tip length to expand linearly with an increase in temperature. However, a time constant that describes a rate of blade tip length growth is generally markedly different than a time constant that describes a rate of shroud expansion, such that the blade tip length generally increases more rapidly.
At least some known aircraft engines activate a clearance control in response to one or more engine operating parameters. Moreover, at least some known aircraft engines activate a clearance control based on an elapsed time relative to a transient engine condition, such as a throttle burst and/or a change in rotor speed. Further, at least some known aircraft engines deactivate a clearance control based on, for example, an aircraft altitude. In addition, other known active clearance controls are based on mathematical models based on data acquired from one or more aircraft engines. However, such controls may not adequately anticipate an increase in fuel flow in order to start shroud expansion prior to the increase in the blade tip length. For example, during flights in which a throttle change is required to climb from one altitude to another, aircraft engine response is conventionally increased based on a predetermined schedule, causing the rotor blades to grow (e.g., lengthen) more rapidly than the surrounding shroud surrounding them, due to mechanical acceleration of the rotor blades. Clearance control systems lag behind the relatively rapid expansion of the blades in an engine speed increase situation, and tolerances must therefore be increased to prevent rub.